Haemophilus influenzae type b (Hib) is the most important cause of endemic bacterial meningitis in infants and has also been identified as the leading cause of acquired mental retardation in the United States. Various investigations clearly indicated that the type-specific capsular polysaccharide of H. influenzae type b is the principal virulent factor in invasive infections by this organism. There is much evidence that serum antibodies to the polysaccharide confer immunity to diseases caused by H. influenzae type b. Vaccination with the purified capsular polysaccharide in adults and children older than two years has been shown to be effective. However, the polysaccharide failed to elicit sufficient serum antibody to provide protection in young infants less than 15 months of age, who are the most susceptible to the infection. It is, therefore, necessary to create a new and efficacious vaccine aimed at this age group. This is particularly important because, although the mortality for Hib-related disease has been reduced to about 5-10% by antibiotic treatment, the morbidity rate remains at 30-50%. (Schneerson et al., Haemophilus Influenzae Type B Polysaccharide-Protein Conjugates: Model for a New Generation of Capsular Polysaccharide Vaccines, New Dev. with Hum. & Vet. Vaccines, 77-94, 1980).